ABSTRACT: Competitive interactions between 2 introduced algae, Caulerpa taxifolia and C. racemosa, were studied in 2 experiments. The first separately evaluated the interspecific and intraspecific effects on both species by manipulatingtheir abundance. The second investigated the fate of fragments of C. taxifolia transplanted into patches of C. racemosa,in contrast to those established on algal turfs. C. racemosa and C. taxifolia had similar temporaltrends in growth but reached different sizes. The overall increase in stolon length at higher densities for both species suggested that positive interactions are very important. However, there was a significant interspecific effect on C. taxifoliastolon length but no effect on C. racemosa. Furthermore, C. taxifolia fragments transplanted into C. racemosa patches showed clear signs of stress with respect to those transplanted into algal turfs: blades became blanched and eroded.Overall, the results of this study suggest that growth increases with density augmentation, therefore suggesting that invasive characteristics of the species may increase with the time of colonisation. Both species tend to spread faster at higherdensity. Overall, where both species co-occur, we predict that C. racemosa would be the favoured species as regards the outcome of competition.